CN102097588A - Method for preparing molecular junction by polydimethylsiloxane stencil printing - Google Patents

Abstract

A method for preparing molecular junctions by polydimethylsiloxane template printing, cleaning the Si substrate and performing photolithography to prepare the lower electrode, preparing a molecular self-assembled film on the surface of the Au lower electrode, and preparing a polydimethylsiloxane template , to prepare a polydimethylsiloxane stamp, prepare the upper electrode, and then print the upper electrode to obtain a cross molecular junction. The invention provides a non-destructive preparation method for the upper electrode of an organic molecular layer cross molecular junction, which avoids the short circuit of the upper and lower electrodes or evaporation deposition caused by a small amount of metal particles penetrating into the inter-molecular pores when the Au film is directly deposited on the organic molecular layer. Disadvantages of thermally induced ablation damage. The method of the invention effectively avoids serious uncertainty between metal/molecular contact interfaces.

Description

A method for preparing molecular knots by polydimethylsiloxane template printing

technical field

The invention belongs to the technical field of electronic devices, relates to molecular electronic device technology, in particular to a method for preparing molecular junctions by polydimethylsiloxane template printing.

Background technique

In the field of molecular electronic devices, it is one of the trends in the development of ultra-miniaturization of electronic circuits in the future to realize the measurement and control of the photoelectric properties of several molecular aggregates or single-layer molecular thin films and the high integration and micronanoization of their devices. The basic idea of building molecular electronic devices is to prepare molecular junctions with a metal-molecule-metal structure. It is still a great challenge to construct micro-nano molecular junctions cheaply and efficiently. At present, a preparation method with simple equipment, low cost and high efficiency is to use PDMS template printing method to prepare molecular junctions. It mainly uses the PDMS stamp with the required micro-nano pattern as the medium for the transfer of the upper metal electrode, so that it slowly contacts and adheres to the organic molecular layer on the lower electrode to form a molecular junction. Since PDMS is an elastic material, it has a great influence on the yield of the prepared device, and it is easy to cause problems such as deformation, adhesion, and short circuit of the upper and lower electrodes of the graphic structure under the action of gravity and surface tension. Usually, the molecular junction prepared by stencil printing method has a thickness of about 50-250nm as a functional thin film medium. For monolayer thin films (thickness below 10nm), the probability of success is very low, so further improvement is needed to obtain better performance. properties of monolayer devices.

Contents of the invention

Aiming at the shortcomings of the above-mentioned prior art, the present invention provides a method for preparing molecular junctions by polydimethylsiloxane template printing, which solves the serious uncertainty problem between metal/molecular contact interfaces.

The method steps of the present invention are as follows.

(1) Cleaning the Si substrate

On the upper and lower sides of the pure Si (100) substrate, a 200-300 nm thick SiO ₂ layer is grown at 1100-1200 ° C by dry oxygen oxidation, and the oxidation time is 4-6 hours. Dip the cut Si substrate into a piranha solution at 50-80°C, take it out after 10-20 minutes, and then immerse it in deionized water, acetone and alcohol in sequence and clean it with ultrasonic waves for 3-5 minutes respectively.

It should be noted that the piranha solution is a mixture of pure H ₂ SO ₄ and H ₂ O ₂ with a mass concentration of 30% in a volume ratio of 4:1. The material reacts violently, and the operator should pay attention to the protection of the body (goggles, lab coats, rubber gloves are required, and the operation should be carried out in a chemical fume hood).

(2) Photolithography

Spin-coat positive resist S1813 on the Si substrate treated in step (1), with a spin-coating amount of 0.7-3μm/cm ² , spread the glue at a speed of 3-4 thousand rpm for 30-60 seconds, and then place it on a heating plate Soft-bake at 110-120°C for 30-60s, take it out and cool to room temperature naturally. Then use PGMEA (1,2-Propanediol monomethyl ether acetate1,2-propylene glycol methyl ether acetate) or EGMEA (Methoxyethyl acetate-2-methoxyethyl acetate) edge removal solvent to remove the photoresist on the edge of the Si substrate . Next, align and cover the pre-designed lower electrode mask on the Si substrate for exposure for 60-90 s. Then pre-bake the Si substrate for 180-300s, the temperature is controlled at 110-120°C, and then naturally cool to room temperature, take it out and develop it in a positive photolithographic developer for 30-60s, and finally wash it with deionized water and dry it.

(3) Prepare the lower electrode

Evaporate and deposit a 5-10nm thick Cr film adhesion layer on the Si substrate after the above photolithography, and then evaporate and deposit an 80-100nm thick Au film at a rate of 0.3-0.5nm/s on the basis of it, the purpose is to make the deposition The Au electrode is more firmly bonded to the substrate. Clean the photoresist and irrelevant impurities on the substrate with acetone (cleaning can be done by ultrasonic vibration for 5-15 s). After taking it out, wash it with flowing acetone, absolute ethanol and deionized water in sequence, and obtain a prominent Cr-Au thin film pattern after drying to form the lower electrode.

(4) Preparation of molecular self-assembled films on the surface of the Au bottom electrode

The above Si substrate with the Au lower electrode was cleaned with absolute ethanol, and dried, and then the Si substrate was soaked in the target molecule in absolute ethanol or tetrahydrofuran (THF, tetrahydrofuran) solution for 22 to 26 hours, the solution concentration A molecular self-assembled film is formed on the surface of the Au film, and the absolute ethanol or tetrahydrofuran solution of the target molecule is contained in a glass container. The target molecule is n-dodecanethiol, 1-decanethiol or n-octyl mercaptan.

(5) Preparation of polydimethylsiloxane (PDMS) template

Spin-coat photoresist negative resist SU-8 on another same Si substrate treated in step (1), uniformize the glue at a speed of 3-4 thousand rpm for 70-100 s, and then place it on a heating plate at 60-60 s. Soft bake at 80°C for 120-150 s, then raise the temperature to 80-100°C for 300-500 s, take it out and cool it to room temperature naturally, then use PGMEA or EGMEA edge removal solvent to remove the photoresist on the edge of the Si substrate remove. Next, align and cover the pre-designed upper electrode mask on the substrate, and expose for 80-100s. Then hard-bake the substrate at 60-80°C for 60-120s, raise the temperature to 80-100°C, then hard-bake for 120-150s, and then cool naturally to room temperature. Put the substrate in the negative film developing solution and develop for 80-100s. Finally, the substrate is washed with deionized water to remove impurities and dried to obtain the desired template pattern.

(6) Preparation of polydimethylsiloxane (PDMS) stamp

Paste the Si substrate produced in step (5) on the bottom of a glass petri dish. Mix PDMS and curing agent at a volume ratio of (8-12): 1, stir evenly, pour the mixture of PDMS and curing agent into an open container and make the height of the mixture 3-5mm, then put PDMS and curing agent Put the open container of the mixture of reagents into a closed container to evacuate to make the vacuum degree ≤10 ³ Pa, so as to remove the air bubbles mixed into the PDMS polymer during the stirring process. Take out the open container containing the mixture of PDMS and curing agent, put it in a drying box and let it stand at 65-75°C for 15-24 hours, the PDMS will be cured, and cut a PDMS block with the same size as the Si substrate with a blade. And remove the PDMS block (you can use a pair of tweezers to clamp the edge of the cut PDMS block to release it from the mold and separate it from the container) to get the PDMS stamp.

The curing agent is polysilicone potting glue.

(7) Prepare the upper electrode

Cover the PDMS stamp obtained in the previous step with a thin aluminum sheet (thickness 0.1-1mm) with a small opening in the middle, leaving only a rectangular area in the middle where the Au electrode needs to be deposited through the small opening (the rectangular area can reveal the complete shape of the upper electrode ). A layer of Au film with a thickness of 15-25 nm is vacuum evaporated and deposited on the PDMS stamp through the small opening, and the deposition rate is 0.3-0.5 nm/s. Before deposition, it is necessary to cool the PDMS stamp with liquid nitrogen to reduce the thermal adhesion effect between the metal and PDMS during the deposition process. Control the cooling temperature at -170°C to -180°C for 10 to 15 minutes, and then deposit after cooling . After the deposition is completed, wait for the temperature of the substrate to rise to room temperature naturally, open the vacuum chamber and take out the PDMS stamp with the gold film for the next step of printing.

(8) Upper electrode printing

Place the PDMS stamp with the gold film on the glass slide (put the side without the Au electrode in contact with the slide glass), and move the Au electrode on the PDMS stamp (upper electrode) under the optical microscope by moving the slide glass. ) is in contact with the Au electrode (lower electrode) with a self-assembled film on the Si substrate prepared in step (4), so that the upper electrode overlapping area overlaps the lower electrode overlapping area, and the molecular junction area of the upper electrode (that is, as shown in Figure 3 The four parallel strips of the upper electrode shown) cross contact with the molecular junction region of the lower electrode (ie, the four parallel strips of the lower electrode shown in Figure 3). Due to the wettability of the elastic PDMS template, the lighter Si substrate was naturally adsorbed on the PDMS template. Then, the device (PDMS stamp on glass slide and Si substrate prepared in step (4)) was immediately placed in a vacuum chamber and cooled to -150°C to -180°C by liquid nitrogen for 10 to 15 minutes. When it naturally rises to room temperature, the vacuum chamber is opened, the PDMS stamp is peeled off, and the upper electrode is printed on the silicon substrate, thus obtaining a cross molecular junction. Use a tungsten needle tip dipped in a gallium indium droplet to pierce through the contact area between the upper electrode overlapping area and the lower electrode overlapping area to form an ohmic contact.

The optoelectronic properties of the obtained molecular junctions were tested with a semiconductor parameter analyzer. The test system consisted of a variable temperature vacuum volt-ampere characteristic test bench, the prepared Si substrate with crossed molecular junctions, a semiconductor parameter analyzer and a data acquisition computer. The electrical characteristics of the device are tested by cyclic voltammetry, and the number of cycles can be determined as required. Place the sample on the temperature-variable vacuum volt-ampere characteristic test bench, connect it to the chip pad of the molecular junction through a thin gold wire, and lead it out of the vacuum chamber through the vacuum electrode lead, connect it to a semiconductor parameter analysis tester, and test the volt-ampere of the corresponding molecular junction characteristic curve. If the molecular junction has been short-circuited or disconnected before the test, the test electrode needs to be switched to another molecular junction, and the above actions should be repeated (note: the molecular junction prepared in the present invention is addressable, and each molecular junction can be repeated testing). When it is necessary to test the temperature-variable volt-ampere characteristics of molecular junction devices, it is necessary to control the temperature of the sample stage in the vacuum chamber, and obtain the molecular junction electrical volt-ampere characteristic curve under the condition of no light in the temperature range of 90K to 300K through liquid nitrogen cooling.

The invention provides a non-destructive preparation method of an organic molecular layer intersecting the molecular junction upper electrode, by adjusting the deposition rate and controlling the deposition time, the Au film is quickly deposited on the PDMS stamp, and then the molecular junction upper electrode is prepared by means of an elastic template printing method, avoiding When the Au film is directly deposited on the organic molecular layer, the disadvantages caused by the short circuit of the upper and lower electrodes or the thermal ablation damage caused by evaporation deposition due to the infiltration of a small amount of metal particles into the intermolecular pores are eliminated. The metal-molecule-metal structure molecular junction prepared by the method of the present invention provides a simple, effective and low-cost test platform for studying the charge transport characteristics of molecular electronic devices, and it effectively avoids the gap between metal/molecular contact interfaces. serious uncertainty. Compared with the conventional mercury drop or scanning probe microscope tip method, the intersecting molecular junction prepared by the present invention can also realize the addressing function, and this addressable crossing molecular junction provides an opportunity to obtain photoelectric switches and transistors driven by low-frequency AC voltage. a simple method. In addition, the stencil printing technology of the present invention provides a technical path for developing novel molecular junction devices with characteristics such as switches, transistors, and light sensing.

Description of drawings

Fig. 1 is the mask design pattern of upper electrode;

Fig. 2 is the mask design pattern of lower electrode;

Figure 3 is a schematic diagram of the device structure of the molecular junction;

Fig. 4 is a schematic diagram of the structure of a thin aluminum sheet;

Fig. 5 is the electrical volt-ampere characteristic curve of the molecular junction prepared in embodiment 1;

In the figure: 1 upper electrode overlapping area; 2 lower electrode overlapping area; 3 gallium indium droplet; 4 Si substrate, 5 upper electrode molecular junction area, 6 lower electrode molecular junction area, 7 thin aluminum sheet, 8 small opening.

Detailed ways

Example 1

The steps of the method for preparing molecular knots by polydimethylsiloxane template printing are as follows.

(1) Cleaning the Si substrate

On the upper and lower sides of the pure Si (100) substrate, a 250 nm thick SiO ₂ layer was grown at 1150 ° C by dry oxygen oxidation method, and the oxidation time was 5 hours. Dip the cut-sized Si substrate into the piranha solution at 65°C, take it out after 15 minutes, and then immerse it in deionized water, acetone and alcohol successively and clean it with ultrasonic wave for 4 minutes respectively.

It should be noted that the piranha solution is a mixture of pure H ₂ SO ₄ and H ₂ O ₂ with a mass concentration of 30% in a volume ratio of 4:1. The material reacts violently, and the operator should pay attention to the protection of the body (goggles, lab coats, rubber gloves are required, and the operation should be carried out in a chemical fume hood).

(2) Photolithography

Spin-coat the positive resist S1813 on the Si substrate treated in step (1), with a spin-coating amount of 1.8 μm/cm ² , uniform the glue at a speed of 3.5 thousand revolutions/min for 45 seconds, and then place it on a heating plate at 115°C for soft coating. Bake for 45s, take it out and cool to room temperature naturally. Then use PGMEA (1,2-Propanediol monomethyl ether acetate) edge removal solvent to remove the photoresist on the edge of the Si substrate. Next, align and cover the pre-designed lower electrode mask on the Si substrate for exposure for 75 s. Then the Si substrate was pre-baked for 240s, the temperature was controlled at 115°C, and then cooled to room temperature naturally. After taking it out, it was developed in a positive resist developer for 45s, and finally cleaned with deionized water and dried.

(3) Prepare the lower electrode

Evaporate and deposit a 7.5nm-thick Cr film adhesion layer on the Si substrate after the above photolithography, and then evaporate and deposit a 90nm-thick Au film at a rate of 0.4nm/s on the basis of it, in order to make the deposited Au electrode stronger combined with the substrate. The photoresist and irrelevant impurities on the substrate were cleaned with acetone. After taking it out, wash it with flowing acetone, absolute ethanol and deionized water in sequence, and obtain a prominent Cr-Au thin film pattern after drying to form the lower electrode.

(4) Preparation of molecular self-assembled films on the surface of the Au bottom electrode

The above Si substrate with the Au lower electrode was cleaned with absolute ethanol, and dried, and then the Si substrate was soaked in the absolute ethanol solution of the target molecule for 24 hours, the solution concentration was 1.5 mmol/L, and the Au A molecular self-assembled film is formed on the surface of the film, and the absolute ethanol solution of the target molecule is contained in a glass container. The target molecule is n-dodecanethiol.

(5) Preparation of polydimethylsiloxane (PDMS) template

Spin-coat photoresist negative resist SU-8 on another same Si substrate treated in step (1), uniformize the glue at a speed of 3.5 thousand rpm for 85 s, and then place it on a heating plate at 70 ° C for 135 s , and then raise the temperature to 90°C for soft baking for 400 s, take it out and cool it to room temperature naturally, and then remove the photoresist on the edge of the Si substrate with PGMEA edge removal solvent. Next, align and cover the pre-designed upper electrode mask on the substrate, and expose for 90s. Then hard-bake the substrate at 70°C for 90s, raise the temperature to 90°C and then hard-bake for 135s, and then naturally cool to room temperature. The substrate was developed in a negative film developer for 90 s. Finally, the substrate is washed with deionized water to remove impurities and dried to obtain the desired template pattern.

(6) Preparation of polydimethylsiloxane (PDMS) stamp

Paste the Si substrate produced in step (5) on the bottom of a glass petri dish. Mix PDMS and curing agent at a volume ratio of 10:1, stir evenly, pour the mixture of PDMS and curing agent into an open container and make the height of the mixture 4mm, and then open the container containing the mixture of PDMS and curing agent The container is placed in a closed container and vacuumed so that the vacuum degree is ≤10 ³ Pa, so as to remove the air bubbles mixed into the PDMS polymer during the stirring process. Take out the open container containing the mixture of PDMS and curing agent, put it in a drying box and let it stand at 70°C for 20h, the PDMS will be cured, use a blade to cut a PDMS block with the same size as the Si substrate, and remove the PDMS block (use a pair of tweezers to clamp the edge of the cut PDMS block to release it from the container), and obtain a PDMS stamp.

The curing agent is polysilicone potting compound, Dow Corning Sylgard® 186 silicone elastomer.

(7) Prepare the upper electrode

Cover the PDMS stamp obtained in the previous step with a thin aluminum sheet with a small opening in the middle, and only leave a rectangular area in the middle where the Au electrode needs to be deposited through the small opening (the rectangular area can expose the complete shape of the upper electrode). A 20 nm thick Au film was vacuum evaporated and deposited on the PDMS stamp through the small opening at a deposition rate of 0.4 nm/s. Before deposition, the PDMS stamp needs to be cooled with liquid nitrogen to reduce the thermally induced adhesion effect between the metal and PDMS during the deposition process. The cooling temperature is controlled at -175°C for 12 minutes, and the deposition is performed after cooling. After the deposition is completed, wait for the temperature of the substrate to rise to room temperature naturally, open the vacuum chamber and take out the PDMS stamp with the gold film for the next step of printing.

(8) Upper electrode printing

Place the PDMS stamp with the gold film on the glass slide (put the side without the Au electrode in contact with the slide glass), and move the Au electrode on the PDMS stamp (upper electrode) under the optical microscope by moving the slide glass. ) is in contact with the Au electrode (lower electrode) with a self-assembled film on the Si substrate prepared in step (4), so that the upper electrode overlapping area overlaps the lower electrode overlapping area, and the molecular junction area of the upper electrode (that is, as shown in Figure 3 The four parallel strips of the upper electrode shown) cross contact with the molecular junction region of the lower electrode (ie, the four parallel strips of the lower electrode shown in Figure 3). Due to the wettability of the elastic PDMS template, the lighter Si substrate was naturally adsorbed on the PDMS template. Then, this device (PDMS stamp on glass slide and Si substrate prepared in step (4)) was immediately placed in a vacuum chamber and cooled to −165 °C by liquid nitrogen for 12 min. When it naturally rises to room temperature, the vacuum chamber is opened, the PDMS stamp is peeled off, and the upper electrode is printed on the silicon substrate, thus obtaining a cross molecular junction. Use a tungsten needle tip dipped in a gallium indium droplet to pierce through the contact area between the upper electrode overlapping area and the lower electrode overlapping area to form an ohmic contact.

The optoelectronic properties of the obtained molecular junctions were tested with a semiconductor parameter analyzer. The test system consisted of a variable temperature vacuum volt-ampere characteristic test bench, the prepared Si substrate with crossed molecular junctions, a semiconductor parameter analyzer and a data acquisition computer. The electrical characteristics of the device are tested by cyclic voltammetry, and the number of cycles can be determined as required. Place the sample on the temperature-variable vacuum volt-ampere characteristic test bench, connect it to the chip pad of the molecular junction through a thin gold wire, and lead it out of the vacuum chamber through the vacuum electrode lead, connect it to a semiconductor parameter analysis tester, and test the volt-ampere of the corresponding molecular junction characteristic curve. If the molecular junction has been short-circuited or disconnected before the test, the test electrode needs to be switched to another molecular junction, and the above actions should be repeated (note: the molecular junction prepared in the present invention is addressable, and each molecular junction can be repeated testing). When it is necessary to test the temperature-variable volt-ampere characteristics of molecular junction devices, it is necessary to control the temperature of the sample stage in the vacuum chamber, and obtain the molecular junction electrical volt-ampere characteristic curve under the condition of no light in the temperature range of 90K to 300K through liquid nitrogen cooling, as shown in Figure 5 Show.

Example 2

The steps of the method for preparing molecular knots by polydimethylsiloxane template printing are as follows.

(1) Cleaning the Si substrate

On the upper and lower sides of the pure Si (100) substrate, a 300 nm thick SiO ₂ layer was grown at 1200 ° C by dry oxygen oxidation method, and the oxidation time was 6 hours. Dip the cut-sized Si substrate into piranha solution at 80°C, take it out after 10 minutes, and then immerse it in deionized water, acetone and alcohol successively and clean it with ultrasonic wave for 5 minutes respectively.

It should be noted that the piranha solution is a mixture of pure H ₂ SO ₄ and H ₂ O ₂ with a mass concentration of 30% in a volume ratio of 4:1. The material reacts violently, and the operator should pay attention to the protection of the body (goggles, lab coats, rubber gloves are required, and the operation should be carried out in a chemical fume hood).

(2) Photolithography

Spin-coat positive resist S1813 on the Si substrate treated in step (1), with a spin-coating amount of 3 μm/cm ² , spread the glue at a speed of 4,000 rpm for 60 seconds, and then put it on a heating plate and bake it at 120°C for soft drying 30s, take it out and let it cool down to room temperature naturally. Then use EGMEA (Methoxyethyl acetate-2-methoxyethyl acetate) edge removal solvent to remove the photoresist on the edge of the Si substrate. Next, align and cover the pre-designed lower electrode mask on the Si substrate for exposure for 90 s. Then the Si substrate was pre-baked for 300s, the temperature was controlled at 110°C, and then cooled to room temperature naturally. After taking it out, it was developed in a positive photolithographic developer for 60s, and finally cleaned with deionized water and dried.

(3) Prepare the lower electrode

Evaporate and deposit a 10nm thick Cr film adhesion layer on the Si substrate after the above photolithography, and then evaporate and deposit a 100nm thick Au film at a rate of 0.5nm/s on the basis of it, in order to make the deposited Au electrode more firm bonded to the substrate. The photoresist and irrelevant impurities on the substrate were cleaned with acetone (cleaned by ultrasonic vibration for 10 s). After taking it out, wash it with flowing acetone, absolute ethanol and deionized water in sequence, and obtain a prominent Cr-Au thin film pattern after drying to form the lower electrode.

(4) Preparation of molecular self-assembled films on the surface of the Au bottom electrode

The above Si substrate with the Au lower electrode was cleaned with absolute ethanol, and dried, and then the Si substrate was soaked in a tetrahydrofuran (THF, tetrahydrofuran) solution of the target molecule for 26 hours, and the solution concentration was 2 mmol/L , A molecular self-assembled film is formed on the surface of the Au film, and the tetrahydrofuran solution of the target molecule is contained in a glass container. The target molecule is 1-decanethiol.

(5) Preparation of polydimethylsiloxane (PDMS) template

Spin-coat photoresist negative resist SU-8 on another same Si substrate treated in step (1), uniformize the glue at a speed of 4,000 rpm for 100 s, and then place it on a heating plate at 80°C for soft baking for 120 s , then raise the temperature to 100°C for soft baking for 300 s, take it out and cool it down to room temperature naturally, and then use EGMEA edge removal solvent to remove the photoresist on the edge of the Si substrate. Next, align and cover the pre-designed upper electrode mask on the substrate, and expose for 100s. Then hard-bake the substrate at 80°C for 60s, raise the temperature to 100°C, then hard-bake for 120s, and then cool naturally to room temperature. The substrate was developed in a negative film developing solution for 100 s. Finally, the substrate is washed with deionized water to remove impurities and dried to obtain the desired template pattern.

(6) Preparation of polydimethylsiloxane (PDMS) stamp

Paste the Si substrate produced in step (5) on the bottom of a glass petri dish. Mix PDMS and curing agent at a volume ratio of 12:1, stir evenly, pour the mixture of PDMS and curing agent into an open container and make the height of the mixture 5mm, and then open the container containing the mixture of PDMS and curing agent The container is placed in a closed container and vacuumed so that the vacuum degree is ≤10 ³ Pa, so as to remove the air bubbles mixed into the PDMS polymer during the stirring process. Take out the open container containing the mixture of PDMS and curing agent, put it in a drying oven and let it stand at 75°C for 15 hours, the PDMS will be cured, cut a PDMS block with the same size as the Si substrate with a blade, and remove the PDMS block (use a pair of tweezers to clamp the edge of the cut PDMS block to release it from the container), and obtain a PDMS stamp.

The curing agent is polysilicone potting compound, Dow Corning Sylgard® 186 silicone elastomer.

(7) Prepare the upper electrode

Cover the PDMS stamp obtained in the previous step with a thin aluminum sheet with a small opening in the middle, and only leave a rectangular area in the middle where the Au electrode needs to be deposited through the small opening (the rectangular area can expose the complete shape of the upper electrode). A 25 nm thick Au film was vacuum evaporated and deposited on the PDMS stamp through the small opening at a deposition rate of 0.5 nm/s. Before deposition, the PDMS stamp needs to be cooled with liquid nitrogen to reduce the heat-induced adhesion effect between the metal and PDMS during the deposition process. The cooling temperature is controlled at -180°C for 10 minutes, and the deposition is performed after cooling. After the deposition is completed, wait for the temperature of the substrate to rise to room temperature naturally, open the vacuum chamber and take out the PDMS stamp with the gold film for the next step of printing.

(8) Upper electrode printing

Place the PDMS stamp with the gold film on the glass slide (put the side without the Au electrode in contact with the slide glass), and move the Au electrode on the PDMS stamp (upper electrode) under the optical microscope by moving the slide glass. ) is in contact with the Au electrode (lower electrode) with a self-assembled film on the Si substrate prepared in step (4), so that the upper electrode overlapping area overlaps the lower electrode overlapping area, and the molecular junction area of the upper electrode (that is, as shown in Figure 3 The four parallel strips of the upper electrode shown) cross contact with the molecular junction region of the lower electrode (ie, the four parallel strips of the lower electrode shown in Figure 3). Due to the wettability of the elastic PDMS template, the lighter Si substrate was naturally adsorbed on the PDMS template. Then, this device (PDMS stamp on glass slide and Si substrate prepared in step (4)) was immediately placed in a vacuum chamber and cooled to −180 °C by liquid nitrogen for 10 min. When it naturally rises to room temperature, the vacuum chamber is opened, the PDMS stamp is peeled off, and the upper electrode is printed on the silicon substrate, thus obtaining a cross molecular junction. Use a tungsten needle tip dipped in a gallium indium droplet to pierce through the contact area between the upper electrode overlapping area and the lower electrode overlapping area to form an ohmic contact.

Example 3

The steps of the method for preparing molecular knots by polydimethylsiloxane template printing are as follows.

(1) Cleaning the Si substrate

On the upper and lower sides of the pure Si (100) substrate, a 200nm thick SiO ₂ layer was grown at 1100° C. by dry oxygen oxidation, and the oxidation time was 4 hours. Dip the cut Si substrate into the piranha solution at 50°C, take it out after 20 minutes, and then immerse it in deionized water, acetone and alcohol successively and clean it with ultrasonic wave for 3 minutes respectively.

It should be noted that the piranha solution is a mixture of pure H ₂ SO ₄ and H ₂ O ₂ with a mass concentration of 30% in a volume ratio of 4:1. The material reacts violently, and the operator should pay attention to the protection of the body (goggles, lab coats, rubber gloves are required, and the operation should be carried out in a chemical fume hood).

(2) Photolithography

Spin-coat positive resist S1813 on the Si substrate treated in step (1), with a spin-coating amount of 0.7 μm/cm ² , uniformly coat the glue at a speed of 3,000 rpm for 30 seconds, and then place it on a heating plate at 110°C for soft coating. Bake for 60s, take it out and cool to room temperature naturally. Then use PGMEA (1,2-Propanediol monomethyl ether acetate) edge removal solvent to remove the photoresist on the edge of the Si substrate. Next, align and cover the pre-designed lower electrode mask on the Si substrate for exposure for 60 s. Then the Si substrate was pre-baked for 180s, the temperature was controlled at 120°C, and then cooled to room temperature naturally. After taking it out, it was developed in a positive resist developer for 30s, and finally cleaned with deionized water and dried.

(3) Prepare the lower electrode

Evaporate and deposit a 5nm thick Cr film adhesion layer on the Si substrate after the above photolithography, and then evaporate and deposit an 80nm thick Au film at a rate of 0.3nm/s on the basis of it, in order to make the deposited Au electrode more firm bonded to the substrate. The photoresist and irrelevant impurities on the substrate were cleaned with acetone (cleaned by ultrasonic vibration for 15 s). After taking it out, wash it with flowing acetone, absolute ethanol and deionized water in sequence, and obtain a prominent Cr-Au thin film pattern after drying to form the lower electrode.

(4) Preparation of molecular self-assembled films on the surface of the Au bottom electrode

The above Si substrate with the Au lower electrode was cleaned with absolute ethanol, and dried, and then the Si substrate was soaked in the absolute ethanol solution of the target molecule for 22 hours, the solution concentration was 1 mmol/L, and the Au A molecular self-assembled film is formed on the surface of the film, and the absolute ethanol solution of the target molecule is contained in a glass container. The target molecule is n-octyl thiol.

(5) Preparation of polydimethylsiloxane (PDMS) template

Spin-coat photoresist negative resist SU-8 on another same Si substrate treated in step (1), uniformize the glue at a speed of 3,000 rpm for 70 s, and then place it on a heating plate at 60°C for soft baking for 150 s , and then raise the temperature to 80°C for soft baking for 500 s, take it out and cool it to room temperature naturally, and then use PGMEA edge removal solvent to remove the photoresist on the edge of the Si substrate. Next, align and cover the pre-designed upper electrode mask on the substrate, and expose for 80s. Then hard-bake the substrate at 60°C for 120s, raise the temperature to 80°C and then hard-bake for 150s, and then naturally cool to room temperature. The substrate was developed in a negative film developer for 80 s. Finally, the substrate is washed with deionized water to remove impurities and dried to obtain the desired template pattern.

(6) Preparation of polydimethylsiloxane (PDMS) stamp

Paste the Si substrate produced in step (5) on the bottom of a glass petri dish. Mix PDMS and curing agent at a volume ratio of 8:1, stir evenly, pour the mixture of PDMS and curing agent into an open container and make the height of the mixture 3mm, and then open the container containing the mixture of PDMS and curing agent The container is placed in a closed container and vacuumed so that the vacuum degree is ≤10 ³ Pa, so as to remove the air bubbles mixed into the PDMS polymer during the stirring process. Take out the open container containing the mixture of PDMS and curing agent, put it in a drying oven at 65°C for 24 hours, and the PDMS will be cured. Use a blade to cut a PDMS block with the same size as the Si substrate, and remove the PDMS block (use a pair of tweezers to clamp the edge of the cut PDMS block to release it from the container), and obtain a PDMS stamp.

The curing agent is polysilicone potting compound, Dow Corning Sylgard® 186 silicone elastomer.

(7) Prepare the upper electrode

Cover the PDMS stamp obtained in the previous step with a thin aluminum sheet with a small opening in the middle, and only leave a rectangular area in the middle where the Au electrode needs to be deposited through the small opening (the rectangular area can expose the complete shape of the upper electrode). A 15 nm thick Au film was vacuum evaporated and deposited on the PDMS stamp through the small opening at a deposition rate of 0.3 nm/s. Before deposition, the PDMS stamp needs to be cooled with liquid nitrogen to reduce the heat-induced adhesion effect between the metal and PDMS during the deposition process. The cooling temperature is controlled at -170°C for 15 minutes, and the deposition is performed after cooling. After the deposition is completed, wait for the temperature of the substrate to rise to room temperature naturally, open the vacuum chamber and take out the PDMS stamp with the gold film for the next step of printing.

(8) Upper electrode printing

Place the PDMS stamp with the gold film on the glass slide (put the side without the Au electrode in contact with the slide glass), and move the Au electrode on the PDMS stamp (upper electrode) under the optical microscope by moving the slide glass. ) is in contact with the Au electrode (lower electrode) with a self-assembled film on the Si substrate prepared in step (4), so that the upper electrode overlapping area overlaps the lower electrode overlapping area, and the molecular junction area of the upper electrode (that is, as shown in Figure 3 The four parallel strips of the upper electrode shown) cross contact with the molecular junction region of the lower electrode (ie, the four parallel strips of the lower electrode shown in Figure 3). Due to the wettability of the elastic PDMS template, the lighter Si substrate was naturally adsorbed on the PDMS template. Then, this device (PDMS stamp on glass slide and Si substrate prepared in step (4)) was immediately placed in a vacuum chamber and cooled to −150 °C by liquid nitrogen for 15 min. When it naturally rises to room temperature, the vacuum chamber is opened, the PDMS stamp is peeled off, and the upper electrode is printed on the silicon substrate, thus obtaining a cross molecular junction. Use a tungsten needle tip dipped in a gallium indium droplet to pierce through the contact area between the upper electrode overlapping area and the lower electrode overlapping area to form an ohmic contact.

Claims (6)

1. a dimethyl silicone polymer mould printing prepares the method that molecule is tied, and it is characterized in that step is as follows:

(1) cleans the Si substrate

At the upper and lower surface of pure Si substrate, use the dry-oxygen oxidation method in 1100～1200 ℃ of thick SiO of growth 200～300 nm ₂Layer, oxidization time 4～6 hours immerses the Si substrate that cuts size in 50～80 ℃ the piranha solution, takes out after 10～20 minutes, immerses successively in deionized water, acetone and the alcohol respectively with ultrasonic waves for cleaning 3～5 minutes again;

(2) photoetching

The positive glue S1813 of spin coating on the Si substrate after step (1) is handled, be placed on subsequently on the heating plate at 110～120 ℃ of following soft baking 30～60s, naturally cool to room temperature after the taking-up, with the trimming solvent photoresist of Si substrate edge is removed again, the bottom electrode mask alignment that next will design is in advance covered 60～90 s that expose on the Si substrate, to dry by the fire 180～300s before the Si substrate then, temperature is controlled to be 110～120 ℃, naturally cool to room temperature again, be placed on the 30～60s that develops in the developer for positive photoresist after the taking-up, clean with deionized water at last and drying;

(3) preparation bottom electrode

The Cr adhesion of film layer that hydatogenesis 5～10nm is thick on the Si substrate after the above photoetching, then on its basis with the thick Au film of speed hydatogenesis 80～100nm of 0.3～0.5nm/s, on-chip photoresist and impurity are washed with acetone, after the taking-up, wash successively with the acetone, absolute ethyl alcohol and the deionized water that flow, obtain the Cr-Au Thinfilm pattern given prominence to after the drying, form bottom electrode;

(4) prepare the molecule self-assembled film in the Au lower electrode surface

The Si substrate of the above Au of having bottom electrode is cleaned with absolute ethyl alcohol, and carry out drying, then the Si substrate is immersed in the absolute ethyl alcohol of target molecule or the tetrahydrofuran solution 22～26 hours, solution concentration be 1～2 mM/liter, generate one deck molecule self-assembled film at the Au film surface, the absolute ethyl alcohol of target molecule or tetrahydrofuran solution are contained in the glass container, and target molecule is selected positive lauryl mercaptan, 1-decyl mercaptan or n-octyl mercaptan for use;

(5) preparation dimethyl silicone polymer template

At the identical negative glue SU-8 of spin coating photoetching on the Si substrate that step (1) is handled of another piece, be placed on subsequently on the heating plate in 60～80 ℃ of soft baking 120～150 s, again temperature is raised to 80～100 ℃ of soft baking 300～500 s, naturally cool to room temperature after the taking-up, with the trimming solvent photoresist of Si substrate edge is removed again, the top electrode mask alignment that next will design is in advance also covered on substrate, exposure 80～100s, then substrate is dried by the fire 60～120s down firmly at 60～80 ℃, be warmed up to 80～100 ℃ of then hard baking 120～150s, naturally cool to room temperature again, substrate is placed on the 80～100s that develops in the developer for negative photoresist, use the washed with de-ionized water substrate at last, remove impurity and dry up the die plate pattern of acquisition expectation;

(6) preparation dimethyl silicone polymer seal

The Si substrate that step (5) is made sticks on a glass culture dish bottom, PDMS is pressed (8～12) with curing agent: 1 volume ratio is mixed, after stirring the mixture of PDMS and curing agent being injected open-top receptacle and making the height of mixture is 3～5mm, the open-top receptacle that the mixture of PDMS and curing agent will be housed then is put in the closed container and vacuumizes, and makes vacuum degree≤10 ³Pa, the open-top receptacle of the mixture of PDMS and curing agent is equipped with in taking-up, is placed on the drying baker inherence and leaves standstill 15～24h for 65～75 ℃, and PDMS promptly solidifies, PDMS block identical of scribing with the Si sizes of substrate, and take off this PDMS block, obtain the PDMS seal; Curing agent adopts the poly-organosilicon casting glue;

(7) preparation top electrode

Cover previous step gained PDMS seal with a middle flake aluminum that has osculum, only reserve a rectangular area of intermediate demand deposition Au electrode by this osculum, by this osculum thick Au film of vacuum evaporation deposition one deck 15～25nm on the PDMS seal, deposition rate is 0.3～0.5nm/s; After deposition is finished, treat that substrate temperature is raised to room temperature naturally, open vacuum chamber and take out the PDMS seal that has golden film, carry out next step printing work;

(8) top electrode printing

The PDMS seal that will have golden film places on the slide, the side with the Au electrode is not contacted with slide, under light microscope, by mobile slide, the Au electrode that has self-assembled film on the Si substrate with the preparation of the Au electrode on the PDMS seal and step (4) contacts, the top electrode overlap is ridden on the bottom electrode overlap, and top electrode molecule interface contacts with the interface right-angled intersection of bottom electrode molecule, lighter Si substrate is attracted on the PDMS template naturally, then, immediately this device is placed in the vacuum chamber, also continue 10～15 minutes by cooled with liquid nitrogen to-150 ℃～-180 ℃, when treating that nature is raised to room temperature, open vacuum chamber, the PDMS seal is peeled off, top electrode is printed on the silicon chip, thereby obtain right-angled intersection molecule knot, prick the zone formation ohmic contact that the top electrode overlap contacts with the bottom electrode overlap with the tungsten tip that dips in the gallium indium drop.

2. prepare the method for molecule knot according to the described dimethyl silicone polymer mould printing of claim 1, it is characterized in that in the step (2), the positive glue S1813 of spin coating on the Si substrate after step (1) is handled, the spin coating amount is 0.7～3 μ m/cm ², with the even glue 30～60s of 3～4,000 rev/mins speed.

3. prepare the method for molecule knot according to the described dimethyl silicone polymer mould printing of claim 1, it is characterized in that in the step (2), the trimming solvent adopts 1,2-propylene glycol methyl ether acetate or acetic acid-2-methoxyl group ethyl ester.

4. prepare the method for molecule knot according to the described dimethyl silicone polymer mould printing of claim 1, it is characterized in that in the step (5), the negative glue SU-8 of spin coating photoetching on the identical pure Si substrate of another piece is with the even glue 70～100s of 3～4,000 rev/mins speed.

5. prepare the method for molecule knot according to the described dimethyl silicone polymer mould printing of claim 1, it is characterized in that in the step (5), the trimming solvent adopts 1,2-propylene glycol methyl ether acetate or acetic acid-2-methoxyl group ethyl ester.

6. prepare the method for molecule knot according to the described dimethyl silicone polymer mould printing of claim 1, it is characterized in that in the step (7), before the deposition, with cooled with liquid nitrogen PDMS seal, the control chilling temperature is-170 ℃～-180 ℃, and the time is 10～15 minutes, has cooled off to deposit again.

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